The protocol that keeps billions of devices in sync — explained simply
NTP (Network Time Protocol) is one of the oldest internet protocols still in use. Created in 1985, it has a single job: keep your computer's clock accurate by comparing it to reliable time references over the network.
Every device you own — your phone, laptop, smart TV, Wi-Fi router — uses NTP (or a variant) to stay in sync. Without it, your clock would drift by seconds per day, and modern internet services would break down.
NTP asks a trusted server "what time is it?", accounts for the network delay, and gently adjusts your clock to match — typically within a few milliseconds of the correct time.
You might think "a few seconds off" is no big deal. But many services you rely on daily depend on accurate time:
A typical computer's internal clock (quartz oscillator) drifts by 0.5 to 2 seconds per day. After a month without NTP, your clock could be off by a full minute. After a year: 10+ minutes.
NTP organizes time servers in a hierarchy called strata (plural of stratum). Think of it as layers of trust, where each layer is one step further from the ultimate time source:
Stratum 1 is the most accurate you can reach over the network. Each additional stratum adds a tiny amount of uncertainty, but even Stratum 3 is accurate to within a few milliseconds — more than enough for any everyday use.
NTP doesn't just ask "what time is it?" — it's much cleverer than that. It needs to account for the time it takes for the message to travel across the network.
Every NTP exchange involves 4 timestamps:
From these 4 timestamps, the client can calculate:
By measuring the round-trip time and subtracting the server's processing time, NTP can estimate the one-way network delay — even though it can't measure it directly. This allows sub-millisecond accuracy over the internet.
NTP doesn't suddenly jump your clock forward or backward. Instead, it gently speeds up or slows down your system clock until it matches the correct time. This prevents disruption to running applications that depend on time flowing smoothly.
Only if the offset is very large (typically over 128 ms at startup) will NTP perform an abrupt correction called a step.
Windows uses the Windows Time Service (w32tm). By default, it syncs with time.windows.com — a Microsoft NTP server. In Active Directory environments, domain controllers handle time sync automatically through the domain hierarchy.
macOS syncs with time.apple.com by default. You can check and change this in System Settings > General > Date & Time.
Most Linux distributions use one of three NTP clients:
Both Android and iOS sync time automatically using a combination of NTP and carrier-provided time. This is why your phone almost always shows the correct time.
Use our NTP Tester to see your system's current offset against a Stratum 1 GNSS reference.
NTP is specified in RFC 5905 (NTPv4, published in 2010). The newer security extension, NTS (Network Time Security), is defined in RFC 8915 (2020).
NTP (Network Time Protocol) synchronizes computer clocks over the internet. It automatically corrects your system's time by comparing it to highly accurate reference clocks, keeping all devices within milliseconds of the correct time.
Over the internet, NTP typically achieves accuracy within 1–50 milliseconds. On a local network (LAN), accuracy can reach sub-millisecond levels. Stratum 1 servers connected directly to GNSS receivers achieve microsecond accuracy.
Stratum levels represent the distance from the reference clock. Stratum 0 is the atomic clock or GNSS receiver itself. Stratum 1 servers connect directly to Stratum 0 sources. Stratum 2 servers sync from Stratum 1, and so on up to Stratum 15. Stratum 16 means "unsynchronized".
Yes, most modern operating systems use NTP by default. Windows uses the Windows Time Service (w32tm), macOS uses timed, and Linux typically uses chrony, systemd-timesyncd, or ntpd. However, these defaults may not always be optimally configured.
The NTP Pool Project is a worldwide cluster of volunteer-operated NTP servers. When your system connects to pool.ntp.org, it's automatically directed to a nearby server. Organizations like RDEM Systems contribute servers to this pool.
See your system's current offset against a Stratum 1 GNSS reference. Instant results, no installation required.
A service by RDEM Systems · See also: NTP diagnostics